Mechanical behavior assessment of FPSO mid-ship repaired stiffened corroded side panels under combined loading

Abstract

Hull plating thickness reduction due to corrosion generates a significant decrease in the structure ultimate load capacity, which may compromise the vessel’s integrity. In offshore oil and gas production units demobilization to a shipyard is unfeasible, so reparation must be performed locally and efficiently. This article presents an experimental - numerical investigation to assess the performance of an alternative in situ repair methodology for FPSO mid-ship corroded side panels subjected to common types of loads. The repair technique consists in the inclusion, with a single-pass welding, of an intermediate light longitudinal steel stiffener on the plating exposed to considerable thickness reduction, to reestablish the load carrying capacity, thus extending its operational life. A set of nine mid-ship panels of a VLCC (very large crude carrier) converted into an FPSO (floating production storage and offloading) platform, on a scale of 1:3.5, are fabricated and subjected to uniaxial compression. The panels portray the corrosion free intact condition, two levels of thickness reduction (25% and two with 58%) and two repair light stiffener profile dimensions for each level of thickness loss. A finite element model (FEM) is developed and firstly employed to compare and validate the experimental results for uniaxial compression. The FEM is subsequently employed to evaluate other typical loading combinations encountered in hull structures, including lateral pressure, uniaxial compression and shear stresses. The results show that the inclusion of an intermediate light longitudinal steel stiffener may satisfactorily restore the plate and consequently the panel load carrying capacity, allowing additional hull plating thickness reduction.